var oHtml = document.documentElement;
getFont();
window.onresize = function(){

	getFont();

}
function getFont(){
	var screenWidth = oHtml.clientWidth;
    if(screenWidth <= 320){

        oHtml.style.fontSize = '50px';
    }else if(screenWidth >= 640){

        oHtml.style.fontSize = '100px';
    }else{

        oHtml.style.fontSize = screenWidth/(640/100)+'px';

    }  	
}